A Wide variety of mechanical, hydraulic and pneumatic exercise devices are currently available. Typically these devices are for the strengthening of musculature, and may provide unilateral or bilateral action, and various types of resistive force both on the extension stroke and the return stroke. Most commercially available exercising devices possess characteristics that focus on maximizing efficiency of muscular contraction but create a risk of injury to the operator, because the principles of joint reaction forces and ligament strains induced by those muscular contractions have not been understood, or have been overlooked or ignored. The use of such devices may be significantly harmful when used in certain medically prescribed rehabilitation programs and strength training programs.
Most commercially available rehabilitation and exercising devices employ a stack of weights that provide resistance in stepwise amounts selected by the operator/user. They are typically arranged to provide resistance to one specific muscle group, for example the quadriceps muscle group on the front of the thigh. The amount of muscle force exerted to move the weight stack is not constant because more force is required to initiate weight movement (to overcome the inertia of rest) than to maintain motion (inertia of motion). Once the weight is put in motion, the changes in speed (acceleration) of the weight stack causes the resistance experienced by the user to change. Thus, as the weight is being accelerated, the resistive force required by the user to further move the weight is decreased rapidly compared to the initial resistive force experienced by the user when beginning to lift or move the weight. The resistive force, however, sharply increases as the movement of the weights is slowed or stopped. This would be the situation when the weights are fully lifted (full extension) or laid to rest (full return). Should the operator suddenly change the direction of applied force or magnitude of force, then higher than predicted stresses are generated across the muscle-tendon unit and the joint surfaces. This is harmful to healing ligaments or injured joint surfaces in the lower extremities.
One example of a mechanical leg exercise device is shown in Graham U.S. Pat. No. 4,884,802. In this mechanical system, in order to change the resistive force, bungee cords must be added or removed from the system. In addition, there is extremely fast rebound because there is nothing to slow the carriage upon return to the rest position. The user in an exercise mode may not experience any discomfort because the muscles are sufficiently strong to stand the rebound shock. However, for a person involved in rehabilitation, such as knee surgery to repair torn ligaments (e.g., anterior cruciate ligament) or patellar surgery, the rebound shock could be significant enough to delay rehabilitation or cause further damage.
Some prior art devices have sought to overcome the variations in muscle forces that occurs throughout the exercise stroke by offering constant resistance devices. These include various types of pneumatic, hydraulic, or motorized resistance mechanisms to dampen the "peaks" and "valleys" of the forces generated during the exercise stroke such as present in the Graham bungee cord sled device.
There is a fundamental difference, however, between hydraulic systems and pneumatic systems. Hydraulic systems involve applying force to a piston which expresses a non-compressive fluid out through a control orifice. These devices tend to be force dependent/rate independent. That is, in order to achieve a certain number of strokes per minute, the force required to express fluid through a given orifice size must be increased. They do not permit easy change of "reps" (i.e., repetitions per minute). The force applied throughout the entire stroke must be relatively constant. Further, once the pressure is released, the system, unless it is a "gravity down" system, will not return to the original rest position. Some systems are bi-acting, that is, the valve is a two way valve rather than a check valve and fluid is merely expressed from one side of a piston to the other and back again during the exercise action so that force must be applied in both strokes. A further disadvantage to hydraulic systems is that the fluid tends to leak from the hydraulic cylinder after a while creating damage to floor covering or a slip hazard around the equipment. A typical example of a hydraulic exercise device would be a hydraulic rowing machine.
Pneumatic systems work against a compressible fluid, air. If they leak, the fluid does not damage the equipment or surrounding area. Upon piston compression of the air, the piston will rebound by the energy that is stored in the compressed air where the air is compressed in a sealed chamber. A variety of exercise devices propose to use accumulators or reservoirs which are in addition to the compressive piston cylinder so that the volume of the gas to be compressed may be varied to provide a variable resistance to the system.
Examples of non-rehabilitative pneumatic exercise devices are shown in Wilmarth (U.S. Pat. No. 4,397,462) and in Keiser (U.S. Pat. No. 4,257,593). Both of these devices are simulants of weight lifting devices for the shoulder and arm musculature. They comprise a horizontal bench and a vertical stand from which is pivoted one or more lever arms which actuate a piston as the source of the pneumatic resistance. Wilmarth calls for a 30-1 volume ratio between his cylinder and his accumulator.
Keiser calls for the use of a pair of interconnecting reservoirs which contain a liquid, the level of which can be adjusted to adjust the air volume through a normally closed, complex valving system. The device is disclosed to be bi-lateral so that individual arms may be exercised independently of each other or may be operated 90 degrees out of phase.
Additionally, most prior art exercise devices are inappropriate for lower extremity rehabilitation in physical therapy programs because they require muscle groups to contract in an isolated manner (i.e. either the quadriceps or the hamstring muscle group contracts), rather than provide mechanisms which require opposing muscle groups to contract simultaneously, i.e. co-contraction of both the agonistic and antagonistic muscle groups. For example, both Wilmarth and Keiser are directed to exercising isolated muscle groups, one group at a time. They do not encourage co-contraction of opposing muscle groups during any part of the exercising stroke.
Finally, prior art devices such as Keiser U.S. Pat. No. 4,257,593 teach away from employing inertia of motion to the advantage of the user once movement is initiated. Both Keiser and Wilmarth involve gravity forces in their design and use.
Neither of the above cited prior art devices specifically address the problems faced by a user who has a fragile knee joint (or other damaged lower extremity condition) and who must strengthen the musculature and ligaments surrounding the weakened or injured area in a manner which does not further aggravate their condition. In the prior art devices, injury may occur because the forces are not reduced, or adjustable to the proper level selected by these devices, at joint positions at which the ligaments or cartilage surfaces are most susceptible to damage.
For example, the position of the knee during muscle contraction is very important in determining ligament and joint reaction forces. Patellofemoral joint pain, one of the most common musculoskeletal problems encountered in our active society, occurs because the stresses generated through tendons, soft tissues, bone, or cartilage surfaces often exceed their biologic tolerance. These stresses are highest when the muscle contraction or strain occurs in deep knee flexion (knee bent) and lowest when the knee is near terminal extension (knee straight). Since joint reaction forces on the patellofemoral joint are highest with the knee fully bent, low resistance would be needed to prevent injury at that end of the exercise stroke. A prior art constant resistance device set for that low level would not be effective beyond initial extension of the knee.
In another situation strains on commonly injured ligaments, most notably the anterior cruciate ligament, are highest when isolated quadriceps activity occurs when the knee is near extension (0 to 40 degrees). It is desirable to rehabilitate these injuries with the knee in flexion and to do so while simultaneously contracting both the quadricep and hamstring muscle groups (co-contraction).
A paradox exists, therefor, because rehabilitation of injured or repaired ligaments in flexion may be harmful to the patellofemoral joint, and rehabilitation of the patellofemoral joint in extension to prevent patellofemoral pain may be harmful to injured ligaments in the knee.
Accordingly there is a need in the art to provide a rehabilitation device for lower extremity injury or surgery patients which allows for simultaneous co-contraction of opposing muscle groups in order to replicate the physiological functions of concentric, eccentric, and closed kinetic chain exercises (i.e. to simulate the activities of running, climbing, jumping and squatting). There is also a need in the art for a rehabilitative device which also includes the ability to utilize momentum to enhance muscle rehabilitation and improve coordination and endurance that has features of variably-controlled resistance that respect tolerances of biological tissues at various joint angles during both the beginning (concentric) and ending (eccentric) phases of the exercising stroke so that injured or weakened joint surface is not subjected to harmful stresses at the critical stages of joint movement.